Porcelain enamelling



United States Patent PORCELAIN ENAMELLING Marco James Cramer, Royal Oak, Mich., assignor to Parker Rust Proof Company, Detroit, Mich., a corporation of Michigan No Drawing. Application June 15, 1954 Serial No. 437,004

12 Claims. (prim-6.14)

The present invention relates generally to vitreous enamelling, and, more particularly, to an improved metallic base stock for vitreous enamels, a method for making such base stock and an enamelled metal article thereof, and an enamelled article produced thereby.

In the past, vitreous enamels have been applied to metallic bases by complicated and expensive procedures. The enamelled coatings produced have been fragile, subject to chipping and cracking and generally not as satisfactory as could be desired. The exceptionally pleasing vitreous coatings have not enjoyed as wide application as would be expected from their wear resistance, chemical resistance and other advantageous properties. The prior enamelled coatings have been particularly susceptible to loosening from their base when the metal is flexed, twisted or struck by impact. It has long been desired to overcome these shortcomings of vitreous coatings.

The usual practice has been to coat the metal surface with metallic nickel and cobalt, apply a primer coating of an enamel containing nickel and cobalt compounds, and then superimpose one or more finish layers of clear or opacified enamels. Many unsuccessful attempts. have been made to devise a single coat process wherein a single enamel coating is fired directly on themetallic surface. In such prior one-coat processes, various special alloy compositions and metal preparation treatments have been proposed including methods for expelling carbon from the base metal, the provision of electrodeposited or thermally deposited layered pure metallic coatings on the base metal, oxide coatings, and the use of acid etches, etc. None of the proposed metal preparing or conditioning expedients have afforded commercially-acceptable enamel adhesion. The usual two-coat procedure is prohibitively expensive for many applications, and moreover, suffers from the periodic scarcity of nickel and cobalt.

The principal object of this invention is to providea new base stock which evidences. improved adhesion when coated with vitreous enamel. coatings.

Another important object is to. provide an easily-controlled method of preparing metallic surfaces, particularly ferrous surfaces such as ordinary carbon steel, to receive vitreous enamel coatings.

A further object is to provide a new-and improved vitreous enamelled article which is less fragile and more resistant to impact, torsion and deformation forces.

An additional object is to provide a. less expensive method for preparing base. stock for vitreous enamelling and which does not require theuse of scarce nickel an cobalt.

Other objects and advantages will appear in the following detailed description when taken in its entirety.

In accordance with the present. invention, it has nowbeen unexpectedly found thatcertain composite coatings containing compounds of arsenic, antimony and/or bismuth and heat-induced metal oxide convert metal surfaces such as those offerrous metals, including ordinary 'ice carbon steels, and alloy steels, into a new and improved base stock for vitreous enamels.

The present invention is based on the discovery. that a composite combination. coating containing arsenic, antimony or bismuthcompounds and heat-induce oxide, and mostly the latter, is surprisingly effective. as a base for vitreous enamel. The presence of the arsenic, anti: mony or bismuth compounds is believed to. exert a modifying influence during the heat-induced oxidation which produces a final compositecoating, which evidences greatly improved adhesion for the enamel. While. at present the greatest commercial significance of the. combination coating of this invention is on ferrous. surfaces, such as enamelling iron, the modified oxide coatings, are. useful also on low-carbon steels, alloy steels and other metals. Broadly stated, the method of. this invention. comprises the steps. of forming a thin adherent coating of arsenic, antimony and/ or bismuth compounds. on the metal surface, such a coating having a weight not exceeding about three grams per square foot of surface area, and heating the resulting coating in, an oxidizing atmosphere to form an oxide coating containing arsenic, antimony and/ or bismuth components. A metal having a surface at least of a ferrous metal. and such ferrous. surface having such a coating thereon is the improved. base stock of this invention.

The improved base stock of this invention: may. be used as the base. stock for the conventional.- two-coat. vitreous enamel finishes including thoseemployingnickel and cobalt compounds,.or it may be employed in a proc-' ess employing a single vitreous-enamel coat. While improved adhesion is shown with either process,the greatest.

advantage is gained when the base stock of this invention is given but a. single enamel coat. With the improved base stock, a single, clear, opaque, or colored enamel.

one-coat system of thisinvention, the number of coating.

and firing steps are reduced; time, labor, and materials are saved; and' the possibility of rejects, due to imperfectcoating and firing also is reduced by a factor of atleast one-half.

In forming the arsenic, antimony and/or bismuthcoatings, an aqueousv acidic solution of one. or more of these: compounds of these metals is utilized. The compound utilized is not critical, and anysoluble compound may be utilized if a slightly acidified solution, of. the. desired. strength can be formed. With arsenic, the most convenient soluble compound is arsenic pentoxide (As 0 which is appreciably soluble in water. Arsenic trioxide (Asgo may also be utilized if desired. These come. pounds form, respectively, arsenic acid (H AsO and? arsenious acid (H AsO in water. Arsenic trioxide, when dissolved in water and oxidized by a suitable oxidizing agent. such as. sodium chlorate, forms arsenic acid directly. Arsenites, such as sodium arsenite (Na AsO orthoarsenites. such as NaH AsO and arsenates such as sodium arsenate (Na H-A O JH' O) and other watersoluble salts of the oxygen acids of arsenic also may be: utilized, if desired. If a salt-type compound. utilized hasabasic reaction in aqueous solution, a smallamount of acidmay be utilized to produce an acidic'solution. With antimony, the trioxide, the tetraoxide and the. pentoxide' may beutilized in a similar fashion, although the-pentoxide is preferred. As with arseniccompounds, the antimony oxides. form weakly: acid: s'olutionsvih water. Bismuth, however, is more basic in nature and requires 3. the addition of small quantities of acid to form the desired acidic solutions. Any of the soluble bismuth compounds such as the sub-carbonate (BiO) CO may be utilized. Since the compounds of these metals in their higher valence states are more effective in coating metals, it is preferred to incorporate in the solution a small amount of an oxidizing agent such as sodium chlorate, hydrogen peroxide, sodium nitrate, Sitol (sodium nitrobenzene sulfonate), the iodates, periodates, bromates, etc.

With any of the arsenic, antimony and/or bismuth solutions, the concentration may vary considerably depending on the time and temperature of treatment. With the commercially feasible treating temperatures of 110 to 195 F. solutions containing the equivalent of from 2 to 10. grams/liter of As O .Sb O and/or (BiO) CO will produce advantageous coatings in from 1 to 60 minutes, more preferably in from 2 to 30 minutes. When the higher temperatures can be employed slightly weaker solutions will produce good coatings and when lower temperatures can be employed to advantage, stronger solutions will produce equivalent coatings.

The coatings produced from the acidic arsenic, antimony, and/r bismuth solutions are believed not to be metallic in nature. That is, the solutions utilized are non-plating solutions, as distinguished from a plating solution such as a solution of arsenic trichloride, for example. The coatings are amorphous in character and are believed to comprise reaction products, for example, arsenates formed between the base metal and AS205, antimonates formed in a similar fashion, etc. i

The acid content of the solutions is not critical. It is only required that the solution be below neutral (i. e. below pH 7). The solution may be strongly acidified, if desired, although such a condition is not preferred due to undue contamination of the treating solution by solution of the base metal. In most cases, only a few drops of acid will suffic'e. For example, the use of only from one to four or five cc. of concentrated hydrochloric, sulfuric, nitric, or other strong inorganic acid for every one to four liters of solution is entirely adequate. A pH range of from 1.5 to 6.0 yields uniform coatings which markedly improve enamel adhesion.

Generally, it is desirable to utilize an oxidizing agent to increase the rate of coating. It is not fully understood but an oxidizing agent is believed to maintain the arsenic, antimony or bismuth in their highest valence states, in which condition coating action is more rapid. For this use, sodium chlorate has been found especially effective. Other oxidizing agents may be utilized, for example, sodium bromate, sodium nitrate, sodium dichromate, potassium permanganate, potassium persulfate, hydrogen peroxide, Sitol (sodium nitrobenzene sulfonate) and others. The proportion of oxidizing agent is not critical. For general purposes, from 2 to 10 grams of an oxidant per liter of solution will usually be suflicient.

The metal surface should be clean and free of grease and soil in order to insure even distribution and good adherence of the arsenic, antimony and/or bismuth coatings. Otherwise, no special preparation of the metal surface usually is required.

If desired, however, many types of ferrous surfaces which are scaled or corroded may be acid etched, activated or pickled to form a more reactive surface, and insure better adhesion. For this purpose, sulfuric, hydrofluoric, hydrochloric, nitric, and other strong inorganic acids may be utilized. Immersion in solutions of these acids in concentrations of from to 20% for from one to ten minutes at temperatures of 115 to 195 P. will usually be sufficient. For metal surfaces which are badly scaled or corroded, molten salt baths may be utilized to loosen the coating after which the usual pickling operation will efliciently remove the remainder.

. The clean metal surface is immersed in the coating solution for a required time and at a temperature to yield the desired weight of coating. As pointed out above, the base coating should not exceed three grams/ sq. ft. In some cases, the base or modifier coatings weighing less than 5 mg./sq. ft. do not yield consistent adhesion. Better results are obtained with the coatings weighing between 5 and 2,500 mg./sq. ft. Best and most consistent results are obtained with coatings weighing between 5 and 750 mg./sq. ft.

After an arsenic, antimony and/or bismuth coating of the above-described type has been formed, the coated metal surface is ready to be oxidized. The step of firing in an oxidizing atmosphere the article having on its surface one of the above-described oxide-type base coats is considered to be of extreme importance to the success of the invention. It is believed that the character, physical form, weight and distribution of the composite oxide coating containing the compounds of arsenic, antimony or bismuth are determinative, to a large degree, of the adhesion between the metallic base and the vitreous enamel. The oxide coating must be of the heat-induced type and not of the rust type. The oxide coating must be fine-grained and tightly adherent, that is, not scaly or blistered. By heat induced is meant simply that the iron oxide portion of the composite coating is formed as the result of the application of heat in the presence of air or other oxidizing atmosphere including oxygen, water, etc. One suitable procedure for producing such a coating comprises the steps of rinsing the arsenic or other base coating in cold water after its removal from the arsenic solution and immediately introducing the coated, wet article into a furnace provided with an oxidizing atmosphere at a temperature above about ll00 F. If the base-coated article cannot be fired immediately in the oxidizing furnace, it is best to dry the article with water-absorbing solvent such as acetone before storage in order to reduce rusting and the formation of other types of corrosion thereon.

As will be appreciated, the amount of heat-induced metallic oxide coating which is formed on the surface of the arsenic, antimony or bismuth coated article is dependent on the temperature employed, the time of firing and the nature of the furnace atmosphere. As the time and temperature and oxygen content of the atmosphere are increased, the weight of heat-induced oxide which is formed also increases. Typically suitable conditions for the firing step are temperatures between 1100 and 1900 F. At 1100 F., as much as thirty minutes are required to produce a coating evidencing increased adhesion for enamel, whereas at 1900 F. more than 50 seconds produces a blistered, scaly coating of no value on most metal surfaces. Temperatures between about 1200 F. and 1550 F. are more practical. A temperature of 1350 F. to 1550 F. permits firing times of between /2 minute to 10 minutes which are commercially suitable for most applications. that time, temperature and firing atmosphere can be varied to produce coatings of the type described above and of a weight within the ranges given below.

As pointed out above, the final composite oxide coat ing comprises mostly heat-induced iron oxide. When the combined coating is correctly prepared, the evidence is clear that adhesion of vitreous enamel is greatly improved with total composite oxide coatings having a weight totaling not more than 15 grams/sq. ft. of which not more than 3 grams/sq. ft. is arsenic, antimony or bismuth compounds. In some cases, consistently good adhesion is not obtained with total coating weights below 0.25 gram/sq. ft. -Total coating weights of 0.4 to 12.0 grams/sq. ft. with modifier between 5 and 2,500 m'g./sq. ft. yield more consistent results, and entirely satisfactory adherence of the enamel coatings is obtained when the total coating weight is between 2.5 and 10.0 grams/sq. ft. Commercially acceptable coatings are obtained with the lighter total coating weights of from It will be understood- 2.5ito grams/ sq. ft. of surface area with the modifying agent. in the range of from 5 to 750 mg./sq. ft. It is to be clearly understood that variations within the broad ranges of 5 mg. to 3 grams/sq. ft. of the arsenic, antimony or bismuth coatings and in the range of 0.25 to 15 5 grams for the total coating are permissible and can be independently controlled. The most desirable coatings from an over-all standpoint, however, are obtained when the weights-of each constituent of the composite oxide coating are maintained within the preferred ranges given above.

The enamel which is applied over the improved base stock of this invention is not critical and may be any conventionally applied over the metal of the base stock. Suitable porcelain enamels include the alkali-aluminum fiuoborosilicate glasses which are fired in a temperature range of 1470 F. to 1560 F., and titanium-opacified enamels of these and other types. For the improved one-coat enamel application over the base stock of this invention, the titanium-opacified enamels or alkali-titanium-fluoborosilicate glasses which are fired at 1400 to 1550 F., have been found to be particularly suitable. These and other enamels may contain coloring pigments to produce attractivclycolored enamel-coated articles. The enamel is applied to the improved base stock in any conventional manner such as by dipping or spraying and thev finished article is obtained by properly firing the enamel coating.

The invention will now be described in more specific detail in the examples below.

Example 1 An aqueous acidic solution was prepared having a concentration of 5 grams/liter of arsenic pentoxide and 2.5 grams/liter of sodium chlorate. Three panels of Armco #3 enamelling iron were treated first by immersing for five minutes in a commercial cleaner solution (Parco Cleaner 350) at a temperature of 170 F., rinsed in cold water and finally immersed for five minutes in a bath containing 10% sulfuric acid at 170 F. The panels were then rinsed in cold water and then variously immersed, respectively, for 15 seconds, three minutes and 30 minutes in the above arsenic bath maintained at 170 F. The coatings had a dark, dusty appearance.

The arsenic-coated panels were then stripped in an aqueous solution of chromic acid, the stripped panels rinsed, dried,

and. reweighing showed that the coating weights-were as follows:

(1) 1.087 grams/sq. ft. (2) 2.195 grams/sq. ft.

After oxidizing in an oxidizing atmosphere for three minutes at 1500 F. and'cooling, the other of each pair of panels was sprayed with a single coat of a slip of a titanium-opacified alkali-metal-fluoborosilicate enamel and fired at about 1500 F. Both panels gave excellent adhesion of the enamel. The enamel-coated panels could be flexed, struck with a hammer' and otherwise severely deformed without loosening or separation of the enamel.

Example 2 Various oxidizing agents were utilized in arsenic pentoxide solutions containing 10 grams of the pentoxide in four liters of solution, the oxidizing agent being utilized in a concentration of 10 grams for every four liters of solution. The baths were heated to about 160 F. and the time of immersion was either two minutes or seven minutes at this temperature. In these solutions, Sitol (sodium nitrobenzene sulfonate), sodium nitrite, potassium permanganate, potassium persulfate, sodium nitrate, sodium dichromate and sodium chlorate, all employed in amounts of 10 grams for each four liters of solution, and hydrogen peroxide, 10 cc. (37%.) in four liters, were found to be satisfactory yielding arsenic coatings, which when fired for five minutes at 1500 F. gave a base suitable for a titanium-opacified porcelain enamel (Pemco Neowite 20). In all cases, adhesion of the: porcelain enamel was much better thanan unmodified oxide coating prepared at a similar temperature but without'the arsenic precoating.

Example 3 A series of treating baths were prepared containing considerable variation in the arsenic pentoxide and oxidant (sodium chlorate) concentration. The. time of immersion in these baths was varied from as little as /2 minute to as much as 10 minutes. A wide variation of coating weights was obtained in these experiments, the weights varying between 0.03 and 1.2 grams per square foot in total weight. The composition of the solutions, the time of immersion, the coating weights obtained and the observed enamel adhesion results after the coatings were prefired for five minutes at 1500" F. are listed below:

and reweighed to determine the weight of the arsenic coating. The coating weights, respectively, determined according to this procedure were as follows:

(1) 0.25 gram/Sq. ft. (2) 0.45 gram/sq. ft. (3) 3.65 gram/sq. ft.

prepared and treated in a similar fashion except that one pair was treated 20 minutes and the other 45 minutes, both at 180 F., in a solution containing 2.5 grams/liter of As O and 2.5 grams/liter of sodium chlorate. Weighing and stripping of one panel of each pair in hydrochloric acid inhibited with formaldehyde A5 0 NaOlOa Time Coat. wt., g./sq. it. Adhesion min. to 10 min 0.25 g. to 1.2 g- Excellent.

o 0.25g to 0.75 g Do. 0 0.03 g to 0.6 g D0. .09g to 0.6 g..." Do. 0.075 g. to 0.66 g. Do.

Example 4 A solution was prepared containing 2.5 grams/liter of antimony pentoxide, 2.5 grams/liter of sodium chlorate, and 1.25 ml. of conc. H SO /liter. The resulting solution evidenced a pH of 2. A number of pairs of panels of #2 enamelling iron were prepared as in Example 1 and each pair immersed, respectively, 1 minute, 5 minutes, and 20 minutes in the antimony solution at F. One panel of each pair was rinsed in cold water and immediately placed in a furnace having an oxidizing atmosphere for 4 minutes at 1500 F. The panel of each pair of panels which was not fired was stripped in chromic acid, rinsed, dried and reweighed. The total weight of antimony coatings determined by this procedure was as follows:

(1) 62 mg./sq. ft. (2) 240 mg./sq. ft. (3.) 507 mg./ sq. ft.

There was evidence that the antimony coating was not completely removed by chromic acid. When stripped in '7 inhibited sulfuric acid No. 2 above, showed a total coating weight of 348 mg./sq. ft. The fired panels of the series were coated with enamel as in Example 1. All three coated panels showed excellent adhesion of the enamel.

Example A four liter bath was prepared containing ten grams of bismuth sub-carbonate, fifty grams of sodium chlorate and 5 cc. of concentrated hydrochloric acid (pH 2.8). Two panels of #2 enamelling iron, pre-treated as in Example l and pickled for five minutes at 170 F. in sulfuric acid, were immersed in this bath for 2.5 minutes at 170 F. and received uniform, adherent coatings of a yellowish dusty color. After removal from the bath,

- the panels were rinsed in cold Water and placed in a furnace maintained at 1500 F. In four minutes of firing, the panels received a uniform, blackish coating. When cooled, sprayed with a slip containing an alkali-metal fluoborosilicate glass, and fired, the panels possessed a smooth, attractive enamel coating which was ditficult to chip or loosen, the enamel evidencing excellent adhesion for the oxide-coated iron.

In a similar fashion, a four-liter bath containing five grams of bismuth sub-carbonate, 10 grams of sodium chlorate, and 5 cc. of concentrated sulfuric acid, after four minutes immersion at 170 F, produced a uniform, adherent coating on cold-rolled steel. After oxidizing for four minutes at 1500 F, a composite oxide coating was produced on the cold-rolled steel to which coating porcelain enamel was tightly adherent. A bismuth sub-carbonate solution of the same strength, except that 5 cc. of concentrated hydrochloric acid were utilized instead of sulfuric acid, produced a coating, which, when oxidized four minutes at 1500 F., was black in color. Porcelain enamel adhered equally well to the latter coating.

Example 6 A solution having a pH of 6.5 and containing 5 grams of antimony trioxide (Sb O and 10 grams of sodium chlorate (pH 6.5) in a volume of 4 liters was used to coat panels of #3 enamelling iron. Panels of the enamelling iron immersed, respectively, three and twelve minutes, at 170 F. in this bath received thin, rusty, yellow coatings, and then each oxidized four minutes at 1500 F, produced excellent adhesion of porcelain enamel. When a similar solution, except for the addition of 5 cc. of concentrated sulfuric acid, was utilized to coat #3 enamelling iron, a much thicker, composite coating was obtained. When the panels were immersed for twelve minutes at 170 F. in the acidified solution and then oxidized as before, the thick coating also was of a rusty, yellow color turning blue-black on oxidizing. Adhesion of porcelain enamel to this thick composite coating also was excellent.

Example 7 A stronger antimony pentoxide solution containing 10 grams of antimony pentoxide, 20 grams of sodium chlorate and 5 cc. of sulfuric acid in a volume of four liters produced coatings on #2 enamel iron and cold-rolled steel, which when oxidized for three minutes at 1500 F produced excellent porcelain adhesion.

What is claimed is:

l. A vitreous enamel base stock comprising a metal having a surface at least of a ferrous metal which surface is coated with a coating comprising the combination of the reaction product of contacting said metallic surface with an aqueous acidic solution of a water soluble compound selected from the class consisting of arsenic, antimony and bismuth compounds and a heat-induced oxide, the weight of said reaction product not exceeding about three grams per sq. ft. of surface area and the total coating having a weight not exceeding grams/sq. ft.

2. A vitreous enamel base stock comprising a ferrous metal having on the surface thereof a composite coating consisting essentially of the combination of the reaction product of contacting said metallic surface with an aqueous acidic solution of a water soluble compound selected from the class consisting of arsenic, antimony and bismuth compounds and a heat-induced oxide, the said reaction product portion of said coating having a weight between 5 mg. and about 3 grams per square foot of surface area and the total coating having a weight in the range of 0.25 and 15 grams per square foot.

3. A vitreous enamel base stock consisting essentially of a ferrous metal having on the surface thereof a composite coating consisting essentially of the combination of the reaction product of contacting said metallic surface with an aqueous acidic solution of a water soluble compound selected from the class consisting of arsenic, antimony and bismuth compounds and a heat-induced oxide, the total composite coating having a weight in the range of 0.4 gram and 12 grams per sq. ft. of surface area, and the said reaction product portion thereof being in the range of 5 mg. and 2,500 mg. per sq. ft.

4. A vitreous enamel base stock comprising a ferrous metal having on the surface thereof a composite coating consisting essentially of the combination of the reaction product of contacting said surface with an aqueous acidic solution of a water soluble compound of arsenic and a heat-induced oxide, the total weight of said composite coating being in the range of 0.4 gram and 12 grams per sq. ft. of surface area and the said arsenic reaction product portion thereof being in the range of 5 mg. and 2,500 mg. per sq. ft.

5. A vitreous enamel base stock comprising a ferrous metal having on the surface thereof a composite coating consisting essentially of the combination of the reaction product of contacting said surface with an aqueous acidic solution of a water soluble compound of antimony and heat-induced oxide, the total weight of said composite coating being in the range of 0.4 gram and 12 grams per sq. ft. of surface area and said antimony reaction product portion thereof being in the range of 5 mg. and 2,500 mg. per sq. ft.

6. A vitreous enamel base stock comprising a ferrous metal having on the surface thereof a composite oxide coating consisting essentially of the combination of the reaction product of contacting said surface with an aqueous acidic solution of a water soluble compound of bismuth and heat-induced oxide, the total weight of said composite coating being in the range of 0.4 gram and 12 grams per sq. ft. of surface area and said bismuth reaction product portion thereof being in the range of 5 mg. and 2,500 mg. per sq. ft.

7. A vitreous enamel base stock comprising a ferrous metal having on the surface thereof a composite coating consisting essentially of the combination of the reaction product of contacting said surface with an aqueous acidic solution of a water soluble compound of arsenic and heatinduced oxide, the total weight of said composite coating being in the range of 2.5 grams and 5 grams per sq. ft. of surface area and the said arsenic reaction product portion thereof being in the range of 5 mg. and 750 mg. per sq. ft.

8. A method of forming a vitreous enamel base stock which comprises the steps of forming on the surface of a metal a coating comprising the combination of the reaction product of contacting said metallic surface with an aqueous acidic solution of a water soluble compound selected from the class consisting of arsenic, antimony and bismuth compounds, said coating having a weight in the range of 5 mg. and about 3 grams per sq. ft. of surface area and heating the resulting coated surface in an oxidizing atmosphere at a temperature and for a time sufiicient to produce on said surface a modified, heatinduced oxide coating having a total weight not less than 0.25 gram nor exceeding 15 grams per sq. ft.

9. A method of forming a vitreous enamelled metallic article which comprises the steps of forming on a surface of a metal an adherent coating consisting essentially of the combination of the reaction product of contacting said metallic surface with an aqueous acidic solution of a water soluble compound selected from the class consisting of arsenic, antimony and bismuth compounds, the weight of said coating being in the range of mg. and about 3 grams per sq. ft. of surface area, heating the resulting coated surface in an oxidizing atmosphere at a temperature above 1100" F. to form a modified, heatinduced oxide coating having a total weight in the range of 0.25 and 15 grams per sq. ft. and thereafter firing a vitreous enamel on said coated surface.

10. A method of forming a vitreous enamel base stock which comprises the steps of forming on a ferrous surface an adherent coating consisting essentially of the combination of the reaction product of contacting said metallic surface with an aqueous acidic solution of a water soluble compound selected from the class consisting of arsenic, antimony and bismuth compounds, the weight of said coating being in the range of 5 mg. and 2,500 mg. per sq. ft. of surface area, and heating the resulting coated surface in an oxidizing atmosphere at a temperature above 1100 F. and below 1900 F. for a time suificient to form a modified heat-induced oxide coating having a total weight in the range of 0.4 gram and 12 grams per sq. ft.

11. A vitreous enamel coated metallic article which comprises a ferrous base stock having on its surface an intervening coating comprising the combination of the reaction product of contacting said metallic surface with 10 an aqueous acidic solution of a water soluble compound selected from the class consisting of arsenic, antimony and bismuth compounds and a heat-induced oxide, the Weight of said reaction product not exceeding about 3 grams/sq. ft. of surface area and the total coating having a weight not exceeding 15 grams/ sq. ft.

12. A method of forming a vitreous enamel base stock which comprises the steps of contacting a base stock having a surface at least of a ferrous metal with an acidic aqueous solution of a water soluble compound selected from the class consisting of arsenic, antimony and bismuth compounds, said treatment being for a time and temperature suflicient to deposit an adherent coating Weighing in the range of 5 mg. and 750 mg. per sq. ft. of surface area and heating the resultant coated surface in an oxidizing atmosphere at temperature above 1100 F. and below 1900 F. and for a time sufiicient to produce a modified, heat-induced, oxide coating having a total Weight in the range of 2.5 to 5 grams per sq. ft.

References Cited in the file of this patent UNITED STATES PATENTS 1,583,006 Pierce et al. May 4, 1926 2,070,368 Martin Feb. 9, 1937 2,099,340 Kautz Nov. 16, 1937 2,301,983 Tanner Nov. 17, 1942 2,321,658 Chester June 15, 1943 2,744,843 Craig May 8, 1956 FOREIGN PATENTS 609,327 France May 10, 1926 

8. A METHOD OF FORMING A VITREOUS ENAMEL BASE STOCK WHICH COMPRISES THE STEPS OF FORMONG ON THE SURFACE OF A METAL A COATING COMPRISING THE COMBINATION OF THE REACTION PRODUCT OF CONTACTING SAID METALLIC SURFACE WITH AN AQUEOUS ACIDIC SOLUTION OF A WATER SOLUBLE COMPOUND SELECTED FROM THE CLASS CONSISTING OF ARSENIC, ANTIMONY AND BISMUTH COMPOUNDS, SAID COATING HAVING A WEIGHT IN THE RANGE OF 5 MG. AND ABOUT 3 GRAMS PER SQ. FT. OF SURFACE AREA AND HEATING THE RESULTING COATED SURFACE IN AN OXIDIZING ATMOSPHRE AT A TEMPERATURE AND FOR A TIME SUFFICIENT TO PRODUCE ON SAID SURFACE A MODIFIED, HEATINDUCE OXIDE COATING HAVING A TOTAL WEIGHT NOT LESS THAN 0.25 GRAM NOR EXCEEDING 15 GRAMS PER SQ. FT. 